1. Field of the Invention
The present invention relates generally to lubricant compensator systems for earth-boring bits. More particularly, the present invention relates to lubricant compensator systems for earth-boring bits of the raise-boring and shaft-boring variety.
2. Background Information
Earth-penetrating tools are divided into two broad categories, those designed to form deep, relatively small diameter boreholes, and those designed to form shallow, large diameter boreholes. Earth-boring bits with rolling cutters mounted on cantilevered bearing shafts are often called "rock bits" and are employed in drilling relatively small diameter boreholes for the recovery of petroleum or other hydrocarbons, or to tap geothermal energy sources. The nature of such drilling operations is that the operations are continued until bit life is nearly or completely expended. These rock bits when used in mining operations, such as the drilling of blast holes, generally are not recovered from the borehole until the bit is effectively destroyed. To extend the life of such rock bits, many are provided with lubrication systems that include a pressure compensator to limit the pressure differential existing between the lubricant and the pressure in the borehole resulting from the presence of drilling fluid in the borehole.
In addition to pressure exerted on the bit by drilling fluids, temperatures increase in the lubricant as the bit is exposed to geothermal temperatures and to frictional heat build-up caused during rotation of the bit. These temperature increases cause an increase in the internal pressure of the lubricant, which causes the lubricant to expand. The increase in the internal pressure of the lubricant may also cause the lubricant to "crack" or emit gaseous hydrocarbons. If an earth-boring bit is of the type having positive seals between the cutters and bearing shafts on which the cutters rotate, the internal pressure build-up can be great enough to damage either the pressure compensator diaphragm or the seal between one of the cutters and bearing shaft. A conventional practice to avoid seal or diaphragm damage is to provide a pressure-relief valve in the lubricant reservoir. Such a pressure-relief valve permits release of lubricant upon the internal pressure of the lubricant exceeding a pre-determined maximum. One example of a pressure relief valve is disclosed in U.S. Pat. No. 3,942,596, Mar. 9, 1976, to Millsapps.
Another type of earth-boring bit employs a plurality of rolling cutters, usually in excess of three, arranged to drill relatively large diameter holes for mining applications. These types of earth-boring bits are used for raise-boring or shaft-boring, which result in large diameter boreholes or shafts.
In raise-boring operations, a pilot hole is drilled with a drillstring, a bit is secured to the drillstring and is rotated and pulled upwardly from a pilot hole to bore a vertical or near vertical shaft. Cuttings generated by the boring operations fall down the shaft and are collected at the bottom of the raise shaft. Because drilling operations are conducted upwardly, it is not feasible to provide cooling and lubricating drilling fluid in raise-boring operations. Thus, the bit experiences considerable frictional heat build-up and the resulting increase in the internal pressure of the lubricant. Therefore, in raise-boring operations, the ability to effectively compensate for internal pressure build-up in the lubricant becomes very important to avoid damage to cutter seals.
In shaft boring operations, the bit is secured directly to a drilling machine and is rotated and pushed through the formation to bore shafts. Drilling fluid is commonly used and exerts hydrostatic pressure on the bit, wherein hydrostatic-pressure compensation becomes quite important, and internal-pressure compensation is less important. Thus, shaft boring bits are provided with lubricant compensators similar to those employed in the small diameter rock bits, with only a pressure-relief valve to attempt to prevent damage to cutter seals from internal pressure increases in the lubricant.
Prior-art lubrication systems do not address the need for both hydrostatic-pressure and internal-pressure compensation of lubricant in the cutters of earth-boring bits of the raise- and shaft-boring variety. A need exists, therefore, for a lubricant compensation system that is capable of compensating lubricant in a lubricant reservoir to equalize pressure differentials existing in the lubricant due to both hydrostatic pressure in the borehole and internal pressure of the lubricant resulting from temperature changes in the lubricant.